The present invention relates to a method of making funnel-like tapered apertures at the ends of a capillary tube bore, and more particularly, to a method of making capillary tube splices and couplers for optical fibers.
Optical fibers have a core surrounded by a cladding having a refractive index lower than that of the core. Optical fiber systems employ various types of optical energy transfer devices that connect or couple energy between such fibers. As used herein, the term "optical energy transfer device" refers to devices for coupling energy between fibers that are disposed in orientations such as end-to-end, side-by-side, and the like.
The end-to-end connection of two optical fibers requires precise axial alignment of the fibers to minimize insertion loss. Due to the relatively large core size of multimode fibers, axial alignment of the cores of such fibers has been accomplished without too much difficulty. However, it is very difficult to axially align the cores of single-mode fibers the core diameters of which are typically 7 to 12 microns.
The technique usually employed for connecting two fibers in end-to-end axial alignment requires a precision centering device such as a connector ferrule or a lead-in ferrule for aligning the adjacent endfaces of the two fibers. Such centering devices may consist of capillary tubes formed of glass, ceramic, gemstone, metal, plastic or the like. To be suitable for the alignment of single-mode fibers, the capillary tube must have a precision bore that is no more than a few microns larger in diameter than the fibers that are to be aligned. In one type of centering device, each fiber extends through the bore of a ferrule such that its endface is flush with the ferrule endface; alignment of the two ferrules bring the fibers into axial alignment. Another type of centering device requires the partial insertion of one fiber into a ferrule bore where it abuts the fiber to which it is to be connected. Since the ferrule bores are only slightly larger in diameter than an optical fiber, they have been provided with enlarged, tapered apertures to facilitate insertion of the fibers.
Another type of coupler relies on the evanescent coupling of optical energy between fibers. A preliminary step in the formation of one coupler of this type requires that two or more optical fibers be inserted into a coupler tube having a bore that is preferably just large enough to accommodate the fibers in side-by-side relationship. The tube is collapsed onto the fibers, and the tube midsection is stretched to reduce the diameters of the fiber cores and to reduce the spacing between the cores. A tight fit between fibers and tube during the preliminary assembly step keeps the fibers in proper alignment during subsequent processing steps. A tapered apertures at the end of the tube bore also facilitates the manufacture of this type of coupler.
Various techniques have been used to form the tapered apertures, depending on the type of material used. Glass, which is a particularly suitable ferrule material, is not easily molded or machined to form apertures having tapered end portions. An attempt to have tapered apertures formed by machining with diamond tools resulted in ferrules which were unacceptable in that the tapered apertures were spherically-shaped, i.e. the angle at which the tapered aperture intersected the bore was so great that the aperture did not properly guide a fiber into the bore.
A method of forming a glass fiber alignment ferrule having enlarged, tapered end portions is disclosed in U.S. patent application S.N. 82,680 filed Aug. 7, 1987 (Berkey 9), now U.S. Pat. No. 4,822,389. A precision glass capillary tube is formed by depositing glass particulate material on a cylindrical mandrel, removing the mandrel, consolidating the resultant cylindrical porous preform and drawing it to reduce the aperture diameter to a size that is slightly larger than the diameter of the optical fibers that are to be connected therein. One end of the resultant tube is attached to a source of air pressure, and a flame is directed onto the tube at spaced intervals while the tube is rotated. A bubble is formed at each region of the tube softened by the flame. The tube is scored and severed at the center of each bubble to produce a capillary tube having tapered apertures at each end thereof. Because of the gradual taper of the enlarged aperture end portions, a fiber that is inserted into the end portion is guided by the tapered wall to the central aperture. However, some fiber connectors require the use of sleeves into which the capillary tube must snuggly fit. Forming a tapered aperture end portion by blowing a bubble may also cause the outside diameter of the glass capillary tube to become enlarged so that it does not fit into the sleeve. Grinding the excess glass from the tube surface to provide a uniform diameter throughout its length is a costly step.